Method and device for multistate interferometric light modulation

ABSTRACT

A multi-state light modulator comprises a first reflector  104 . A first electrode  142  is positioned at a distance from the first reflector  104 . A second reflector  14  is positioned between the first reflector  104  and the first electrode  142 . The second reflector  14  is movable between an undriven position, a first driven position, and a second driven position, each having a corresponding distance from the first reflector  104 . In one embodiment, the light modulator has latch electrodes  17  and  143 , which hold the light modulator in a driven state. In another embodiment the latch electrodes  17  and  143  are used to alter the actuation and release thresholds of the light modulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/235,686, titled “METHOD AND DEVICE FOR MULTISTATE INTERFEROMETRICLIGHT MODULATION,” filed Sep. 26, 2005, which claims the benefit of U.S.Provisional Application No. 60/613,891, titled “Systems and Methods forInterferometric Modulation,” filed Sep. 27, 2004. The disclosures of theabove-reference applications are considered part of the disclosure ofthis application and are incorporated by reference herein.

BACKGROUND

1. Field of the Invention

The field of the invention relates to microelectromechanical systems(MEMS).

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF CERTAIN EMBODIMENTS

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

One embodiment includes a light modulator, including a movable reflectorhaving an electrically conductive material. The movable reflector ispositioned between first and second electrodes, and is movable betweenan undriven position, a first driven position, and a second drivenposition. The first driven position is closer to the first electrodethan is the undriven position and the second driven position is fartherfrom the first electrode than is the undriven position. The lightmodulator also includes at least one third electrode adjacent to thefirst electrode, and at least one fourth electrode adjacent to thesecond electrode.

Another embodiment includes a light modulator, including means forreflecting light positioned between first and second means forpositioning the reflecting means. The reflecting means is movablebetween an undriven position, a first driven position, and a seconddriven position. The first driven position is closer to the firstpositioning means than is the undriven position and the second drivenposition is farther from the first positioning means than is theundriven position. The light modulator also includes third means forpositioning the reflecting means adjacent to the first positioningmeans, and fourth means for positioning the reflecting means adjacent tothe second positioning means.

Another embodiment includes a method of driving a MEMS device includingfirst, second, third, and fourth electrodes, and a movable electrodepositioned between the first electrode and the second electrode andconfigured to move to at least two positions therebetween. The methodincludes applying a first voltage potential difference between the firstelectrode and the movable electrode so as to drive the movable electrodeto a position substantially in contact with a dielectric layer. A forceis created attracting the movable electrode towards the dielectriclayer. The method also includes applying a second voltage potentialdifference between the first electrode and the movable electrode and athird voltage potential difference between the second electrode and themovable electrode so as to overcome the force attracting the movableelectrode towards the dielectric layer and to drive the movableelectrode away from the dielectric layer. Also included in the method isapplying a fourth voltage potential difference between the thirdelectrode and the movable electrode, and applying a fifth voltagepotential difference between the fourth electrode and the movableelectrode. The force attracting the movable electrode towards thedielectric layer is based at least in part on the fourth and fifthvoltages.

Another embodiment includes a method of fabricating a multistate lightmodulator. The method includes forming first and second electrodes,forming a movable reflector including an electrically conductivematerial, where the movable reflector is positioned between the firstand second electrodes, and is movable between an undriven position, afirst driven position, and a second driven position. The first drivenposition is closer to the first electrode than is the undriven positionand the second driven position is farther from the first electrode thanis the undriven position. The method also includes forming at least onethird electrode adjacent to the first electrode, and forming at leastone fourth electrode adjacent to the second electrode.

Another embodiment includes a display apparatus including a plurality ofdisplay elements, each of the display elements including a movablereflector including an electrically conductive material, where themovable reflector is positioned between first and second electrodes, andis movable between an undriven position, a first driven position, and asecond driven position. The first driven position is closer to the firstelectrode than is the undriven position and wherein the second drivenposition is farther from the first electrode than is the undrivenposition, at least one third electrode adjacent to the first electrode,and at least one fourth electrode adjacent to the second electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a side cross-sectional view of an exemplary interferometricmodulator that illustrates the spectral characteristics of producedlight.

FIG. 9 is a graphical illustration of reflectivity versus wavelength formirrors of several exemplary interferometric modulators.

FIG. 10 is a chromaticity diagram that illustrates the colors that canbe produced by a color display that includes exemplary sets of red,green, and blue interferometric modulators.

FIG. 11 is a side cross-sectional view of an exemplary multistateinterferometric modulator.

FIGS. 12A-12C are side cross-sectional views of another exemplarymultistate interferometric modulator.

FIGS. 13A-13C are side cross-sectional views of an exemplary multistateinterferometric modulator having latch electrodes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theinvention may be implemented in any device that is configured to displayan image, whether in motion (e.g., video) or stationary (e.g., stillimage), and whether textual or pictorial. More particularly, it iscontemplated that the invention may be implemented in or associated witha variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise of several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. In some embodiments, the layers are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a panel or display array (display) 30. The cross section ofthe array illustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. ForMEMS interferometric modulators, the row/column actuation protocol maytake advantage of a hysteresis property of these devices illustrated inFIG. 3. It may require, for example, a 10 volt potential difference tocause a movable layer to deform from the relaxed state to the actuatedstate. However, when the voltage is reduced from that value, the movablelayer maintains its state as the voltage drops back below 10 volts. Inthe exemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the relaxed or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be relaxed areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putin. After being written, each pixel sees a potential difference withinthe “stability window” of 3-7 volts in this example. This feature makesthe pixel design illustrated in FIG. 1 stable under the same appliedvoltage conditions in either an actuated or relaxed pre-existing state.Since each pixel of the interferometric modulator, whether in theactuated or relaxed state, is essentially a capacitor formed by thefixed and moving reflective layers, this stable state can be held at avoltage within the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Relaxing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias). As is also illustrated in FIG.4, it will be appreciated that voltages of opposite polarity than thosedescribed above can be used, e.g., actuating a pixel can involve settingthe appropriate column to +V_(bias), and the appropriate row to −ΔV. Inthis embodiment, releasing the pixel is accomplished by setting theappropriate column to −V_(bias), and the appropriate row to the same−ΔV, producing a zero volt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding, and vacuum forming. In addition, the housing 41 may be madefrom any of a variety of materials, including but not limited toplastic, metal, glass, rubber, and ceramic, or a combination thereof. Inone embodiment the housing 41 includes removable portions (not shown)that may be interchanged with other removable portions of differentcolor, or containing different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43 which is coupled to a transceiver 47. Thetransceiver 47 is connected to the processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g. filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to the arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a hi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell, including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some implementations control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some cases control programmabilityresides in the array driver 22. Those of skill in the art will recognizethat the above-described optimization may be implemented in any numberof hardware and/or software components and in various configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as support posts. Theembodiment illustrated in FIG. 7D has support post plugs 42 upon whichthe deformable layer 34 rests. The movable reflective layer 14 remainssuspended over the cavity, as in FIGS. 7A-7C, but the deformable layer34 does not form the support posts by filling holes between thedeformable layer 34 and the optical stack 16. Rather, the support postsare formed of a planarization material, which is used to form supportpost plugs 42. The embodiment illustrated in FIG. 7E is based on theembodiment shown in FIG. 7D, but may also be adapted to work with any ofthe embodiments illustrated in FIGS. 7A-7C as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields some portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34 and the busstructure 44. This allows the shielded areas to be configured andoperated upon without negatively affecting the image quality. Thisseparable modulator architecture allows the structural design andmaterials used for the electromechanical aspects and the optical aspectsof the modulator to be selected and to function independently of eachother. Moreover, the embodiments shown in FIGS. 7C-7E have additionalbenefits deriving from the decoupling of the optical properties of thereflective layer 14 from its mechanical properties, which are carriedout by the deformable layer 34. This allows the structural design andmaterials used for the reflective layer 14 to be optimized with respectto the optical properties, and the structural design and materials usedfor the deformable layer 34 to be optimized with respect to desiredmechanical properties.

Embodiments of interferometric modulators described above operate in oneof a reflective state, which produces white light, or light of a colordetermined by the distance between the mirrors 14 and 16, or in anon-reflective, e.g., black, state. In other embodiments, for example,embodiments disclosed in U.S. Pat. No. 5,986,796, the movable mirror 14may be positioned at a range of positions relative to the fixed mirror16 to vary the size of the resonant gap 19, and thus the color ofreflected light.

FIG. 8 is a side cross-sectional view of an exemplary interferometricmodulator 12 that illustrates the spectral characteristics of light thatwould be produced by positioning the movable mirror 14 at a range ofpositions 111-115. As discussed above, a potential difference between arow and column electrode causes the movable mirror 14 to deflect. Theexemplary modulator includes a conductive layer 102 of indium-tin-oxide(ITO) acting as a column electrode. In the exemplary modulator, themirror 14 includes the row conductor.

In one embodiment, a dielectric layer 104 of a material such as alumina(Al₂O₃) is positioned over a layer of chrome that forms a reflectivesurface of the mirror 16. As discussed above with reference to FIG. 1,the dielectric layer 104 prevents shorting and controls the separationdistance between the mirrors 14 and 16 when the mirror 14 deflects. Theoptical cavity formed between the mirrors 14 and 16 thus includes thedielectric layer 104. The relative sizes of items in FIG. 8 have beenselected for purposes of conveniently illustrating the modulator 12.Thus, such distances are not to scale and are not intended to berepresentative of any particular embodiment of the modulator 12.

FIG. 9 is a graphical illustration of reflectivity versus wavelength forthe mirrors 16 of several exemplary optical stacks. The horizontal axisrepresents a range of wavelengths of visible light incident on theoptical stacks. The vertical axis represents the reflectivity of theoptical stack as a percentage of incident light at a particularwavelength. In one embodiment, in which the optical stack does notinclude the dielectric layer 104, the reflectivity of the mirror 16formed of a layer of chrome is approximately 75%. An optical stackincluding a dielectric layer 104 comprising a 100 Å layer of aluminaresults in 65% reflectivity and a dielectric layer 104 comprising a 200Å layer of alumina results in 55% reflectivity. As shown, reflectivitydoes not vary according to wavelength in these particular embodiments.Accordingly, by adjusting the thickness of an Al₂O₃ layer, thereflectivity of the mirror 16 can be controlled consistently across thevisible spectrum to allow specific properties of interferometricmodulators 12 to be selected. In certain embodiments, the dielectriclayer 104 is a layer of Al₂O₃, having a thickness in the range of 50-250Å. In other embodiments, the dielectric layer 104 comprises a thin layerof Al₂O₃, having a thickness in the range of 50-100 Å and a layer ofbulk SiO₂, having a thickness in the range of 400-2000 Å.

As discussed above, the modulator 12 includes an optical cavity formedbetween the mirrors 14 and 16. The characteristic distance, or effectiveoptical path length, L, of the optical cavity determines the resonantwavelengths, λ, of the optical cavity and thus of the interferometricmodulator 12. The resonant wavelength, λ, of the interferometricmodulator 12 generally corresponds to the perceived color of lightreflected by the modulator 12. Mathematically, the distance L=½Nλ, whereN is an integer. A given resonant wavelength, λ, is thus reflected byinterferometric modulators 12 having distances L of ½λ(N=1), λ(N=2),3/2λ(N=3), etc. The integer N may be referred to as the order ofinterference of the reflected light. As used herein, the order of amodulator 12 also refers to the order N of light reflected by themodulator 12 when the mirror 14 is in at least one position. Forexample, a first order red interferometric modulator 12 may have adistance L of about 325 nm, corresponding to a wavelength λ of about 650nm. Accordingly, a second order red interferometric modulator 12 mayhave a distance L of about 650 nm. Generally, higher order modulators 12reflect light over a narrower range of wavelengths and thus producecolored light that is more saturated.

Note that in certain embodiments, the distance, L, is substantiallyequal to the distance between the mirrors 14 and 16. Where the spacebetween the mirrors 14 and 16 comprises only a gas (e.g., air) having anindex of refraction of approximately 1, the effective optical pathlength is substantially equal to the distance between the mirrors 14 and16. In embodiments that include the dielectric layer 104, which has anindex of refraction greater than one, the optical cavity is formed tohave the desired optical path length by selecting the distance betweenthe mirrors 14 and 16 and by selecting the thickness and index ofrefraction of the dielectric layer 104, or of any other layers betweenthe mirrors 14 and 16. In one embodiment, the mirror 14 may be deflectedone or more positions within a range of positions to output acorresponding range of colors. For example, the voltage potentialdifference between the row and column electrodes may be adjusted todeflect the mirror 14 to one of a range of positions in relation to themirror 16. In general, the greatest level of control of the position ofthe mirror by adjusting voltage is near the undeflected position of thepath of the mirror 14 (for example, for smaller deflections, such asdeflections within about ⅓rd of the maximum deflection from theundeflected position of the mirror 14).

Each of a particular group of positions 111-115 of the movable mirror 14is denoted in FIG. 8 by a line extending from the fixed mirror 16 to anarrow point indicating the positions 111-115. Thus, the distances111-115 are selected so as to account for the thickness and index ofrefraction of the dielectric layer 104. When the movable mirror 14deflects to each of the positions 111-115, each corresponding to adifferent distance, L, the modulator outputs light to a viewing position101 with a different spectral response that corresponds to differentcolors of incident light being reflected by the modulator 12. Moreover,at position 111, the movable mirror 14 is sufficiently close to thefixed mirror 16, that the effects of interference are negligible andmodulator 12 acts as a mirror that reflects substantially all colors ofincident visible light substantially equally, e.g., as white light. Thebroadband mirror effect is caused because the small distance L is toosmall for optical resonance in the visible band. The mirror 14 thusmerely acts as a reflective surface with respect to visible light.

As the gap is increased to the position 112, the modulator 12 exhibits ashade of gray as the increased gap distance between the mirrors 14 and16 reduces the reflectivity of the mirror 14. At the position 113, thedistance L is such that the cavity operates interferometrically butreflects substantially no visible wavelengths of light because theresonant wavelength is outside the visible range.

As the distance L is increased further, a peak spectral response of themodulator 12 moves into visible wavelengths. Thus, when the movablemirror 14 is at position 114, the modulator 12 reflects blue light. Whenthe movable mirror 14 is at the position 115, the modulator 12 reflectsgreen light. When the movable mirror 14 is at the non-deflected position1116, the modulator 12 reflects red light.

In designing a display using interferometric modulators 12, themodulators 12 may be formed so as to increase the color saturation ofreflected light. Saturation refers to the intensity of the hue of colorlight. A highly saturated hue has a vivid, intense color, while a lesssaturated hue appears more muted and grey. For example, a laser, whichproduces a very narrow range of wavelengths, produces highly saturatedlight. Conversely, a typical incandescent light bulb produces whitelight that may have a desaturated red or blue color. In one embodiment,the modulator 12 is formed with a distance L corresponding to higherorder of interference, e.g., 2nd or 3rd order, to increase thesaturation of reflected color light.

An exemplary color display includes red, green, and blue displayelements. Other colors are produced in such a display by varying therelative intensity of light produced by the red, green, and blueelements. Such mixtures of primary colors such as red, green, and blueare perceived by the human eye as other colors. The relative values ofred, green, and blue in such a color system may be referred to astristimulus values in reference to the stimulation of red, green, andblue light sensitive portions of the human eye. In general, the moresaturated the primary colors, the greater the range of colors that canbe produced by the display. In other embodiments, the display mayinclude modulators 12 having sets of colors that define other colorsystems in terms of sets of primary colors other than red, green, andblue.

FIG. 10 is a chromaticity diagram that illustrates the colors that canbe produced by a color display that includes two sets of exemplary red,green, and blue interferometric modulators. The horizontal and verticalaxes define a chromaticity coordinate system on which spectraltristimulus values may be depicted. In particular, points 120 illustratethe color of light reflected by exemplary red, green, and blueinterferometric modulators. White light is indicated by a point 122. Thedistance from each point 120 to the point 122 of white light, e.g., thedistance 124 between the point 122 for white and the point 120 for greenlight, is indicative of the saturation of light produced by thecorresponding modulator 12. The region enclosed by the triangular trace126 corresponds to the range of colors that can be produced by mixingthe light produced at points 120. This range of colors may be referredto as the color gamut of the display.

Points 128 indicate the spectral response of another set of exemplarymodulators 12. As indicated by the smaller distance between the points128 and the white point 122 than between points 120 and point 122, themodulators 12 corresponding to the points 128 produce less saturatedlight that do the modulators 12 corresponding to the points 120. Thetrace 130 indicates the range of colors that can be produced by mixingthe light of points 128. As is shown in FIG. 10, the trace 126 enclosesa larger area than does the trace 130, graphically illustrating therelationship between the saturation of the display elements and the sizeof the color gamut of the display.

In a reflective display, white light produced using such saturatedinterferometric modulators tends to have a relatively low intensity to aviewer because only a small range of incident wavelengths is reflectedto form the white light. In contrast, a mirror reflecting broadbandwhite light, e.g., substantially all incident wavelengths, has a greaterintensity because a greater range of incident wavelengths is reflected.Thus, designing reflective displays using combinations of primary colorsto produce white light generally results in a tradeoff between colorsaturation and color gamut and the brightness of white light output bythe display.

FIG. 11 is a side cross-sectional view of an exemplary multistateinterferometric modulator 140 that can produce highly saturated colorlight in one state and relatively intense white light in another state.The exemplary modulator 140 thus decouples color saturation from thebrightness of output white light. The modulator 140 includes a movablemirror 14 that is positioned between two electrodes 102 and 142. Themodulator 140 also includes a second set of posts 18 a that are formedon the opposite side of the mirror 14 as the posts 18.

In certain embodiments, each of the mirrors 14 and 16 may be part of astack of layers defining a reflector or reflective member that performfunctions other than reflecting light. For example, in the exemplarymodulator of FIG. 11, the mirror 14 is formed of one or more layers of aconductive and reflective material such as aluminum. Thus, the mirror 14may also function as a conductor. Similarly, the mirror 16 may be formedof one or more layers of reflective material and one or more layers ofan electrically conductive material so as to perform the functions ofthe electrode 102. Furthermore, each of the mirrors 14 and 16 may alsoinclude one or more layers having other functions, such as to controlthe mechanical properties affecting deflection of the mirror 14. In oneembodiment, the moveable mirror 14 is suspended from an additionaldeformable layer such is described in connection with FIG. 7C.

In one embodiment that includes modulators that reflect red, green, andblue light, different reflective materials are used for modulators thatreflect different colors so as to improve the spectral response of suchmodulators 12. For example, the movable mirror 14 may include gold inthe modulators 12 configured to reflect red light.

In one embodiment, dielectric layers 144 may be positioned on eitherside of the conductor 142. The dielectric layers 144 a and 104advantageously prevent electrical shorts between conductive portions ofthe mirror 14 and other portions of the modulator 140. In oneembodiment, the mirror 16 and the electrode 102 collectively form areflective member.

In the exemplary embodiment, the distance between fixed mirror 16 andthe movable mirror 14 in its undriven position corresponds to theoptical path length L in which the modulator 140 is non-reflective or“black.” In the exemplary embodiment, the optical path length betweenthe fixed mirror 16 and the movable mirror 14 when driven towards thefixed mirror 16 corresponds to the optical path length L in which themodulator 140 reflects white light. In the exemplary embodiment, thedistance between the fixed mirror 16 and the movable mirror 14 whendriven towards the conductor 142 corresponds to the optical path lengthL in which the modulator 140 reflects light of a color such as red,blue, or green. In certain embodiments, the distance between theundriven movable mirror 14 and the fixed mirror 16 is substantiallyequal to the distance between the undriven movable mirror 14 and theelectrode 142. Such embodiments may be considered to be two modulatorspositioned around the single movable mirror 14.

When a first voltage potential difference is applied between the mirror14 and the electrode 102, the mirror 14 deflects towards the mirror 16to define a first optical path length, L, that corresponds to a firstdriven state. In this first driven state, the movable mirror 14 iscloser to the mirror 16 than in the undriven state. When a secondvoltage potential difference is applied between the mirror 14 and theelectrode 142, the mirror 14 is deflected away from the mirror 16 todefine a second optical path length, L, that corresponds to a seconddriven state. In this second driven state, the movable mirror 14 isfarther from the mirror 16 than in the undriven state. In certainembodiments, at least one of the first driven state and second drivenstate is achieved by applying voltage potential differences both betweenthe mirror 14 and the electrode 102 and between the mirror 14 and theelectrode 142. In certain embodiments, the second voltage difference isselected to provide a desired deflection of the mirror 14.

As illustrated in FIG. 11, in the first driven state, the mirror 14deflects to a position indicated by the dashed line 152. In theexemplary modulator 140, the distance between the mirrors 14 and 16 inthis first driven state corresponds to the thickness of the dielectriclayer 104. In the exemplary modulator 140, the mirror 14 acts as abroadband mirror in this position, substantially reflecting all visiblewavelengths of light. As such, the modulator 140 produces a broadbandwhite light when illuminated by broadband white light.

In the second driven state, the mirror 14 deflects to a positionindicated by the dashed line 154. In the exemplary modulator 140, thisdistance corresponds to a color of light, e.g., blue light. In theundriven state, the mirror 14 is positioned as shown in FIG. 11. In theundetected position, the mirror 14 is spaced at a distance from themirror 16 so that substantially no visible light is reflected, e.g., an“off” or non-reflective state. Thus, the modulator 140 defines aninterferometric modulator having at least three discrete states. Inother embodiments, the positions of the movable mirror 14 in the threestates may be selected so as to produce different sets of colors,including black and white, as desired.

In one embodiment, light enters the modulator 12 through the substrate20 and is output to a viewing position 141. In another embodiment, thestack of layers illustrated in FIG. 11 is reversed, with layer 144closest to the substrate 20 rather than layer 102. In certain suchembodiments, the modulator 12 may be viewed through the opposite side ofthe stack from the substrate 20 rather than through the substrate 20. Inone such embodiment, a layer of silicon dioxide is formed on the ITOlayer 102 to electrically isolate the ITO layer 102.

As noted above, having a separate state for outputting white light in amodulator 140 decouples the selection of the properties of the modulatorcontrolling color saturation from the properties affecting thebrightness of white output. The distance and other characteristics ofthe modulator 140 may thus be selected to provide a highly saturatedcolor without affecting the white light produced in the first state. Forexample, in an exemplary color display, one or more of the red, green,and blue modulators 12 may be formed with optical path lengths Lcorresponding to a higher order of interference.

The modulator 140 may be formed using lithographic techniques known inthe art, and such as described above with reference to the modulator 12.For example, the fixed mirror 16 may be formed by depositing one or morelayers of chromium onto the substantially transparent substrate 20. Theelectrode 102 may be formed by depositing one or more layers of atransparent conductor such as ITO onto the substrate 20. The conductorlayers are patterned into parallel strips, and may form columns ofelectrodes. The movable mirror 14 may be formed as a series of parallelstrips of a deposited metal layer or layers (orthogonal to the columnelectrodes 102) deposited on top of posts 18 and an interveningsacrificial material deposited between the posts 18. Vias through one ormore of the layers described above may be provided so that etchant gas,such as xenon diflouride, can reach the sacrificial layers. When thesacrificial material is etched away, the deformable metal layers areseparated from the fixed layers by an air gap. A highly conductive andreflective material such as aluminum may be used for the deformablelayers, and these strips may form row electrodes in a display device.The conductor 142 may be formed by depositing posts 18 a over themovable mirror 14, depositing an intervening sacrificial materialbetween the posts 18 a, depositing one or more layers of a conductorsuch as aluminum on top of the posts 18 a, and depositing a conductivelayer over the sacrificial material. When the sacrificial material isetched away, the conductive layer can serve as the electrode 142 whichis separated from the mirror 14 by a second air gap. Each of the airgaps provides a cavity in which the mirror 14 may move to achieve eachof the states described above.

As further illustrated in FIG. 11, in the exemplary modulator 140, theconductive mirror 14 is connected to the row driver 24 of the arraycontroller 22. In the exemplary modulator 140, the conductors 102 and142 are connected to separate columns in the column driver 26. In oneembodiment, the state of the modulator 140 is selected by applying theappropriate voltage potential differences between the mirror 14 and thecolumn conductors 102 and 142 according to the method described withreference to FIGS. 3 and 4.

FIGS. 12A-12C illustrate another exemplary interferometric modulator 150that provides more than two states. In the exemplary modulator 150, themirror 16 includes both a reflective layer and a conductive layer so asto perform the function of the electrode 102 of FIG. 11. The conductivelayer 142 can also be protected by a second dielectric layer 144 a andsupported by a support surface 148 that is maintained some distanceabove the movable mirror 14 through a second set of supports 18 a.

FIG. 12A illustrates the undriven state of the modulator 150. As withthe modulator 140 of FIG. 11, the mirror 14 of the exemplary modulator150 of FIGS. 12A-12C is deflectable towards the dielectric layer 104(e.g., downwards), as in the driven state illustrated FIG. 12B, and isdeflectable in the reverse or opposite direction (e.g., upwards), asillustrated in FIG. 12C. This “upwardly” deflected state may be calledthe “reverse” driven or actuated state and the “downwardly” deflectedstate may be called the “forward” driven or actuated state.

As will be appreciated by one of skill in the art, this reverse drivenstate can be achieved in a number of ways. In one embodiment, thereverse driven state is achieved through the use of an additional chargeplate or conductive layer 142 that can electrostatically pull the mirror14 in the upward direction, as depicted in FIG. 12C. The exemplarymodulator 150 includes what is basically two interferometric modulatorspositioned symmetrically around a single movable mirror 14. Thisconfiguration allows each of the conductive layer of the mirror 16 andthe conductive layer 142 to attract the mirror 14 in oppositedirections.

In certain embodiments, the additional conductive layer 142 may beuseful as an electrode in overcoming stictional forces that may developwhen the mirror 14 comes in close proximity, or contacts, the dielectriclayer 104. These forces can include van der Waals or electrostaticforces, as well as other possibilities as appreciated by one of skill inthe art. In one embodiment, a voltage pulse applied to the conductivelayer of the mirror 16 may send the movable mirror 14 into the forwarddriven state of FIG. 12B. Similarly, the next voltage pulse can beapplied to the conductive layer 142 to attract the movable mirror 14away from the mirror 16. In certain embodiments, such a voltage pulseapplied to the conductive layer 142 can be used to accelerate therecovery of the movable mirror 14 back to the undriven state illustratedin FIG. 12A from the forward driven state illustrated in FIG. 12B bydriving the movable mirror 14 towards the reverse driven state. Thus, incertain embodiments, the modulator 150 may operate in only two states,the undriven state of FIG. 12A and the forward driven state of FIG. 12B,and can employ the conductive layer 142 as an electrode to help overcomestictional forces. In one embodiment, the conductive layer 142 may bedriven as described above each time that the modulator 150 changes fromthe reverse driven position of FIG. 12C to the undriven position of FIG.12A.

As will be appreciated by one of skill in the art, not all of theseelements will be required in every embodiment. For example, if theprecise relative amount of upward deflection (e.g., as shown in FIG.12C) is not relevant in the operation of such embodiments, then theconductive layer 142 can be positioned at various distances from themovable mirror 14. Thus, there may be no need for support elements 11 a,the dielectric layer 144 a, or a separate support surface 148. In theseembodiments, it is not necessarily important how far upward the movablemirror 14 deflects, but rather that the conductive layer 142 ispositioned to attract the mirror 14 at the appropriate time, such as tounstick the modulator 12. In other embodiments, the position of themovable mirror 14 as shown in FIG. 12C, may result in altered anddesirable optical characteristics for the interferometric modulator. Inthese embodiments, the precise distance of deflection of the movablemirror 14 in the upward direction can be relevant in improving the imagequality of the device.

As will be appreciated by one of skill in the art, the materials used toproduce the layers 142, 144 a, and support surface 148 need not besimilar to the materials used to produce the corresponding layers 16,104 and 20. For example, light need not pass through the layer 148.Additionally, if the conductive layer 142 is positioned beyond the reachof the movable mirror 14 in its deformed upward position, then themodulator 150 may not include the dielectric layer 144 a. Additionally,the voltages applied to the conductive layer 142 and the movable mirror14 can be accordingly different based on the above differences.

As will be appreciated by one of skill in the art, the voltage appliedto drive the movable mirror 14 from the forward driven state of FIG.12B, back to the undriven state of FIG. 12A, may be different than thatrequired to drive the movable mirror 14 from the undriven state of FIG.12A to the upward or reverse driven state of FIG. 12C, as the distancebetween the conductive layer 142 and movable mirror 14 may be differentin the two states. Such requirements can depend upon the desiredapplication and amounts of deflection, and can be determined by one ofskill in the art in view of the present disclosure.

In some embodiments, the amount of force or duration that a force isapplied between the conductive layer 142 and the movable mirror 14 issuch that it only increases the rate at which the interferometricmodulator transitions between the forward driven state and the undrivenstate. Since the movable mirror 14 can be attracted to either conductivelayer 142 or the conductive mirror 16, which are located on oppositesides of movable mirror 14, a very brief driving force can be providedto weaken the interaction of movable mirror 14 with the opposite layer.For example, as the movable mirror 14 is driven to interact with fixedconductive mirror 16, a pulse of energy to the opposite conductive layer142 can be used to weaken the interaction of the movable mirror 14 andthe fixed mirror 16, thereby make it easier for the movable mirror 14 tomove to the undriven state. FIGS. 13A-13C are cross-sectional side viewsof an interferometric modulator that is similar to the interferometricmodulator of FIGS. 12A-12C, except for the addition of “latch”electrodes 17 and 143, electrically isolated by dielectric 153, that aredescribed in further detail below. In FIG. 13A, the interferometricmodulator is shown in the undriven position with the moveable mirror 14in a mechanically relaxed state. FIG. 13B shows the interferometricmodulator in a forward actuated state and FIG. 13C shows theinterferometric modulator in a reverse actuated state.

In one embodiment, a relatively low voltage may initially be applied toelectrodes 17 and/or 143, creating a voltage difference between theelectrodes 17 and/or 143 and the moveable mirror 14. In advantageousembodiments of this design, this voltage difference is not of sufficientmagnitude to cause the moveable mirror 14 to deform from the undrivenstate into either the forward actuated state or the reverse actuatedstate, but is sufficient to hold the moveable mirror 14 in the forwardactuated state or reverse actuated state once it is placed in thatstate. Subsequent to the application of the relatively low voltage tothe electrodes 17 and/or 143, an actuation voltage may be applied toelectrode 16 or electrode 142 that creates a voltage difference betweenthe moveable mirror 14 and the electrode 16 or the electrode 142 that isof sufficient magnitude to cause the moveable mirror 14 to move towardsthe electrode 16 or the electrode 142. After the device is actuated orreverse actuated by this applied voltage, the voltage on electrode 16 orthe electrode 142 may be removed. Because of the close proximity of themoveable mirror 14 and the electrodes 17 or 143, the moveable mirror 14is then maintained in the actuate or reverse actuated position by thevoltage difference between the latch electrode 17 or the latch electrode143 and the moveable mirror 14 even though the voltage applied to thelatch electrode 17 or the latch electrode 143 is not high enough toactuate or reverse actuate the device from the undriven initial state.In one embodiment, the voltage applied to latch electrodes 17 and/or 143is in the range of 1-10 volts, while the voltage applied to electrode 16or electrode 142 is in the range of 5-15 volts. It will be appreciatedthat the voltage applied to the latch electrodes 17 and/or 143 could beapplied after the voltage applied to the electrode 16 or the electrode142.

Once the mirror 14 is latched by the latch electrode 17 or 143, thecorresponding electrode 16 or 142, respectively, may be reduced or maybe undriven. This is advantageous, for example, in a drive scheme wherea single interferometric modulator driver may be used to drive multipleinterferometric modulator elements. In some embodiments once an elementis driven to the desired state, the latch electrode 17 or 143 can holdthe interferometric modulator in that state while the driver disconnectsfrom the electrode 16 and/or 142 to drive another interferometricmodulator element, leaving the electrode 16 and/or 142 undriven.

The movement of the movable mirror 14 shown in FIGS. 13A-13C is governedby the electrostatic forces acting upon it. The latch electrodes 17 and143 are additional contributors to the forces acting upon the movablemirror 14. The position of the mirror 14 is, therefore, a function ofthe voltage difference between the mirror 14 and each of the electrodes16, 142, 17 and 143. Accordingly, electrodes 17 and 143 allow foradditional control over the movement of the movable mirror 14.

For example, if a voltage (with respect to the movable mirror 14) isplaced on electrode 17 there will be a corresponding attractive forcebetween the movable mirror 14 and the electrode 17. This force may notbe enough to actuate the mirror 14 itself, but will reduce the voltagebetween the mirror 14 and the electrode 16 required to actuate themirror 14. Thus, placing a voltage on electrode 17 lowers the actuationthreshold. Similarly, the force between the mirror 14 and the electrode17 will reduce the voltage on electrode 16 at which the mirror returnsto the undriven state (release threshold). Indeed, when used as a latch,the voltage on electrode 16 is such that even reducing the voltagedifference between the mirror 14 and the electrode 16 to zero isinsufficient to cause the mirror 14 to return to the undriven state.Similarly, placing a voltage on electrode 143 induces an electrostaticforce between the mirror 14 and electrode 143. This force works againstthe actuation of the mirror downward, and therefore increases theactuation threshold and reduces the release threshold.

The effect of voltages on latch electrodes 17 and 143 on the actuationand release thresholds for electrode 16 are analogous to the effect ofvoltages on latch electrodes 17 and 143 on the reverse actuation andrelease thresholds for electrode 142. The polarity of the effect of thevoltages is, however, reversed. Where a voltage on 17 reduces theactuation threshold and reduces the release threshold of electrode 16,it increases the reverse actuation threshold and increases the releasethreshold of electrode 142. Similarly, where a voltage on 143 increasesthe actuation threshold and increases the release threshold of electrode16, it decreases the reverse actuation threshold and decreases therelease threshold of electrode 142.

Because the voltages of the latch electrodes 17 and 143 can be varied intime and independently, each of the actuation and release thresholds canbe individually manipulated. For example, when the mirror 14 is in theundriven state the voltages on the latch electrodes 17 and 143 willaffect the actuation and reverse actuation thresholds. Once the mirror14 is in either the forward actuated or reverse actuated states, thevoltages on electrodes 17 and/or 143 may be changed to alter the releasethreshold to a desired level.

Such control over the actuation and release thresholds, and thereforethe hysteresis curves of the interferometric modulator allows forcalibration of the thresholds. For example, because of processingvariations, the thresholds of the individual; interferometric modulatorsmay not be as well matched as is desired. Proper use of the latchelectrodes can correct for the mismatch. Additionally, there may be adesired relationship between the actuation/release behavior and thereverse actuation/release behavior of individual interferometricmodulators. For example, it may be desired that the actuation thresholdbe substantially identical to the reverse actuation threshold. As thesebehaviors can be individually manipulated, the desired relationship maybe attained.

In some embodiments, the voltages on electrodes 17 and 143 may becontrolled to tune the position of the mirror 14 in its undriven state.Because of processing uncertainties, the precision of the position ofthe undriven mirror 14 may not be sufficiently controlled. The forcesgenerated between the mirror 14 and the electrodes 17 and 143 may beused to correct the position.

It will be appreciated that the location of the various components shownin FIGS. 13A-13C can be varied widely. For example, the latch electrodesmay extend further toward and/or under the support posts. Additionally,the latch electrodes may be in a plane different than the electrode 16,for example, above or below. In addition, only one, or more than twolatch electrodes could be provided. An important feature is that thelatch electrode(s) be placed in a position that provides the latchfunction. In the embodiment of FIGS. 13A-13C, it is advantageous to haveat least a portion of the latch electrodes placed beneath the pointwhere the moveable material contacts the dielectric.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers. The scope of the invention is indicated by the appended claimsrather than by the foregoing description. All changes which come withinthe meaning and range of equivalency of the claims are to be embracedwithin their scope.

1. A light modulator, comprising: a movable reflector comprising anelectrically conductive material, the movable reflector positionedbetween a first set and a second set of electrodes, the movablereflector being movable between an undriven position, a first drivenposition, and a second driven position, wherein the first drivenposition is closer to the first set of electrodes than is the undrivenposition and wherein the second driven position is farther from thefirst set of electrodes than is the undriven position; wherein the firstset of electrodes drives the movable reflector to the first drivenposition; wherein the second set of electrodes drives the movablereflector to the second driven position; and further comprising a thirdset of electrodes configured to modify at least one of a actuation and arelease threshold of the movable reflector; and a fourth set ofelectrodes configured to modify at least one of a reverse actuation anda reverse release threshold of the movable reflector.
 2. The modulatorof claim 1, wherein the modulator substantially absorbs incident visiblelight when the movable reflector is in the undriven position.
 3. Themodulator of claim 1, wherein the modulator reflects white light whenthe movable reflector is in the first driven position.
 4. The modulatorof claim 1, wherein the modulator selectively reflects light in a rangeof visible wavelengths associated with a color when the movablereflector is in the second driven position.
 5. The modulator of claim 1,wherein the movable reflector remains in the first driven position whenthe voltage potential is removed from the first set of electrodes. 6.The modulator of claim 1, wherein the movable reflector remains in thesecond driven position when the voltage potential is removed from thesecond set of electrodes.
 7. The modulator of claim 1, furthercomprising: a processor that is in electrical communication with atleast one of the first and second electrodes, the processor beingconfigured to process image data; and a memory device in electricalcommunication with the processor.
 8. The modulator of claim 7, furthercomprising a driver circuit configured to send at least one signal to atleast one of the first, second, third and fourth set of electrodes. 9.The modulator of claim 8, further comprising a controller configured tosend at least a portion of the image data to the driver circuit.
 10. Themodulator of claim 7, further comprising an image source moduleconfigured to send the image data to the processor.
 11. The modulator ofclaim 10, wherein the image source module comprises at least one of areceiver, transceiver, and transmitter.
 12. The modulator of claim 7,further comprising an input device configured to receive input data andto communicate the input data to the processor.
 13. The modulator ofclaim 1 wherein the sets of electrodes comprise one to two electrodes.14. The modulator of claim 1 wherein the first set of electrodes and thethird set of electrodes are both used to drive the movable reflector tothe first driven position.
 15. The modulator of claim 1 wherein thesecond set of electrodes and the fourth set of electrodes are both usedto drive the movable reflector to the second driven position.
 16. Alight modulator, comprising: means for reflecting light positionedbetween first and second means for positioning the reflecting means, thereflecting means being movable between an undriven position, a firstdriven position, and a second driven position, wherein the first drivenposition is closer to the first positioning means than is the undrivenposition and wherein the second driven position is farther from thefirst positioning means than is the undriven position; third means formodifying at least one of a actuation and a release threshold of themovable reflector; and fourth means for modifying at least one of areverse actuation and a reverse release threshold of the movablereflector.
 17. The modulator of claim 16, wherein the reflecting meanscomprises a movable reflector comprising an electrically conductivematerial.
 18. The modulator of claim 17, wherein the first, second,third, and fourth positioning means each comprise a set of electrodes.19. The modulator of claim 16 wherein the first means for positioningthe reflecting means and the third means for maintaining the reflectingmeans are both used to drive the reflecting means to the first drivenposition.
 20. The modulator of claim 16 wherein the second means forpositioning the reflecting means and the fourth means for maintainingthe reflecting means are both used to drive the reflecting means to thesecond driven position.
 21. A method of driving a MEMS device comprisingfirst, second, third, and fourth electrodes, and a movable electrodepositioned between the first electrode and the second electrode andconfigured to move to at least two positions therebetween, the methodcomprising: applying a first voltage potential difference between thefirst electrode and the movable electrode so as to drive the movableelectrode to a position substantially in contact with a dielectriclayer, wherein a force is created attracting the movable electrodetowards the dielectric layer; applying a second voltage potentialdifference between the first electrode and the movable electrode and athird voltage potential difference between the second electrode and themovable electrode so as to overcome the force attracting the movableelectrode towards the dielectric layer and to drive the movableelectrode away from the dielectric layer; applying a fourth voltagepotential difference between the third electrode and the movableelectrode; and applying a fifth voltage potential difference between thefourth electrode and the movable electrode, wherein the force attractingthe movable electrode towards the dielectric layer is based at least inpart on the fourth and fifth voltages.
 22. The method claim 21, whereinthe force comprises a stictional force.
 23. A method of fabricating amultistate light modulator, the method comprising: forming first andsecond electrodes; forming a movable reflector comprising anelectrically conductive material, the movable reflector positionedbetween the first and second electrodes, the movable reflector beingmovable between an undriven position, a first driven position, and asecond driven position, wherein the first driven position is closer tothe first electrode than is the undriven position and wherein the seconddriven position is farther from the first electrode than is the undrivenposition; forming at least one third electrode adjacent to the firstelectrode; and forming at least one fourth electrode adjacent to thesecond elelctrode.
 24. The method of claim 23, wherein forming themovable reflector comprises forming a fifth electrode.
 25. A lightmodulator manufactured by the method of claim 23.